1. Field of the Technology
The technology presented herein relates to an image processing apparatus and a method, and particularly to an image forming apparatus and a method forming a latent image for embedding information in an image.
2. Background Art
Since copy machine technologies have been developed in recent years, it is now possible to copy or counterfeit documents without difficulty and this has become a significant social issue. Therefore, a latent image printing technique has been proposed as one of techniques that reveal a latent image when copied so that one can identify a reproduction at a first glance.
Japanese Patent Laying-Open No. 54-074125 provides a printed matter in which a latent image is printed in fine dots (i.e., halftone dots), and a portion around the latent image is printed in coarse dots. Since fine dots are hard to copy, and one can easily recognize the latent image when the printed matter is copied. The technique in Japanese Patent Laying-Open No. 54-074125 utilizes a limit of a reproduction performance of a copying machine. For example, sheets of paper employing a similar technique are used for copies of certificates of residence so that letters of “COPY” or the like may appear when the certificate of residence is copied.
Description will now be given on a latent image with reference to FIGS. 23 and 24. FIG. 23 shows an example of the latent image. In FIG. 23, an image of a letter “A” serving as a latent image is formed of a coarse pattern 21, and a peripheral image is formed of a fine pattern 22 to exhibit the same shade level as the latent image. When a copying machine copies a sheet on which the image of FIG. 23 is printed, a copied matter in FIG. 24 is produced. On the copied matter in FIG. 24, fine pattern 22 is not present, or a shade difference is present between coarse and fine patterns 21 and 22 due to a performance limit of the copying machine so that a letter of “A” embedded as the latent image appears.
Referring to FIGS. 25A-25D, description will now be given on the case where the latent image technique is utilized on sheets for printing an original. An original image shown in FIG. 25A is printed on a sheet bearing a background image containing a latent image (i.e., an image of a letter “A”) shown in FIG. 25B. FIG. 25C shows a result of such printing. On a printed matter in FIG. 25C, the latent image is integral with a background around it, and the background image exhibits a uniform pattern so that a person viewing the printed matter in FIG. 25C considers that the original is printed on a uniform pattern. When the printed matter in FIG. 25C is copied, the latent image appears on the copied matter as already described. Therefore, by employing an image of letters of “COPY” as the latent image, one can easily determine whether the printed matter was produced by copying or not.
However, according to the technique in Japanese Patent Laying-Open No. 54-074125, it is necessary to prepare sheets having a latent image effect before printing the documents in view of an estimated volume of required sheets. When a design of the latent image is changed, new sheets of paper must be prepared, which results in a waste of the printed sheets, and excessively increases a cost. Further, it is impossible to change the letters to be embedded in response to every printing of the document.
Japanese Patent Laying-Open No. 07-231384 has disclosed a technique in which a digital copying machine records a background image containing a latent image in addition to an original image read by a CCD (Charge Coupled Device). The digital copying machine performs internal image processing to overlay the background image containing the latent image on the original image so that it is not necessary to prepare in advance the printed sheets bearing the background image containing the latent image.
Referring to FIGS. 25A-25D, description will now be given on overlaying of the original image and the background image containing the latent image. In the technique of Japanese Patent Laying-Open No. 54-074125, it is necessary to print in advance the background image containing the latent image on sheets of paper. However, according to the technique of Japanese Patent Laying-Open No. 07-231384, the image processing is performed to overlay the original image (see FIG. 25A) on the background image (see FIG. 25B) containing the latent image, and thereby the print output of the image in FIG. 25C is performed so that it is not necessary to prepare in advance the sheets bearing the background image containing the latent image.
The techniques disclosed in Japanese Patent Laying-Open Nos. 54-074125 and 07-231384 utilize the coarse and fine patterns that exhibit the same shade level to the eye. Therefore, the patterns are uniform at a distant view, but a boundary between the coarse and fine patterns becomes conspicuous in a close view, and such a problem occurs that the latent image is readily recognized with the naked eye even when it is not copied.
Accordingly, a technique has been disclosed in Japanese Patent Laying-Open No. 04-170569 for providing a camouflage pattern of an open or hollow form that is independent of a latent image pattern and extends through a background image and the latent image. Since the camouflage pattern is more conspicuous than the boundary between the coarse and fine patterns, it is difficult to recognize the latent image with the naked eye. Further, as compared with the technique of overprinting another pattern, this technique can camouflage the latent image with less cost and time.
Referring to FIGS. 23, 26 and 27, description will now be given on the camouflage pattern. Referring to FIG. 23, coarse pattern 21 and dense (fine) pattern 22 are printed to exhibit the same shade level, and therefore can be recognized as a uniform pattern in a distant view, but a boundary between the coarse and fine patterns becomes conspicuous in a close view due to a difference in density so that the latent image is recognized with the naked eye. In view of this, as shown in FIG. 26, a hollow camouflage pattern 23 is arranged over patterns 21 and 22. Since the camouflage pattern is more conspicuous than the pattern boundary, the latent image can be camouflaged. When the background image containing the latent image thus camouflaged with the pattern is copied, such an effect is ensured that the fine pattern is not copied similarly to the case where the camouflage pattern is not employed, and the latent image appears as shown in FIG. 27
Japanese Patent Laying-Open No. 2003-101762 has disclosed a technique in which one symbol is assigned to one dot pattern, and dot patterns are arranged in combination for embedding information in an image. The dot patterns will now be described with reference to FIGS. 28A, 28B and 29. For example, the dot pattern in FIG. 28A is assigned a symbol of 0, and the dot pattern in FIG. 28B is assigned a symbol of 1. These symbols are combined, and the dot patterns in FIG. 28A or 28B are arranged according to the arrangement or alignment of the combined symbols so that a uniform background pattern 30 can be formed as shown in FIG. 29. Therefore, by combining it with the techniques in Japanese Patent Laying-Open Nos. 54-074125 and 07-231384, a part of the background pattern may be replaced with another pattern that exhibits the same shade level, and thereby it is possible to provide the background image containing the latent image that appears when copied.
However, one can easily recognize a boundary portion between the dot pattern bearing the symbols and the replaced pattern similarly to Japanese Patent Laying-Open Nos. 54-074125 and 07-231384, and a problem that the latent image is visible to the naked eye is not yet overcome. In particular, the pattern formed of arrangement of the dot patterns bearing the symbols exhibits a peculiar pattern, and thus the peculiar pattern becomes more conspicuous.
The camouflage pattern of the Japanese Patent Laying-Open No. 04-170569 may be used for overcoming the problem that the latent image is visible to the naked eye. In this case, the latent image is not conspicuous to the eye, but such a problem arises that the camouflage pattern destroys the dot pattern, and information embedded in the image cannot be read.
As described above, when information is embedded in at least one of the plurality of patterns forming the latent image, a mere combination of the foregoing patent references cannot camouflage the latent image to make it inconspicuous without losing the embedded information.